Approximately eight out of ten of driver seats (hereinafter, called as a “seat”) of commercial vehicles (in particular, large trucks) in European markets are 3-point belt-in seats. The belt-in seat is the one in which a seat belt is not fixed to a vehicle but attached to a seat in itself, and a shoulder strap thereof is fixed not to a B-pillar of a vehicle body but to a backrest of the seat.
The seat belt is attached to the seat in itself as stated above, and thereby, it is possible to wear the seat belt under optimum state compared to a conventional seat belt and to enhance a protection effect owing to the seat belt because the seat belt follows a movement of the seat by an air suspension especially as for a large truck being equipped with the air suspension as a seat suspension.
On the other hand, the belt-in seat in which the seat and seat belt are integrated tends to move forward because a seat back is pulled together with the shoulder strap caused by an inertia force of a passenger at a time of a head-on collision between two cars (hereinafter, called as a “vehicle collision time”). As a result, a belt binding force is lowered compared to a normal seat belt (belt-in cabin), a breast movement amount of the passenger increases, and there is a possibility in which the breast is smashed against a steering wheel.
Besides, a reinforcing structure of a seat preventing falling down of a seat back toward forward at a vehicle collision time is disclosed in Patent Document 1.
This conventional example is the one in which an engagement unit including an engagement claw is provided at a sidewall of a seat back facing a vehicle sidewall, and a bracket including a wire is provided at an inner wall of a door to face the engagement unit, and the falling down of the seat back at the vehicle collision time is prevented by the engagement between the engagement claw and the wire.